It is known that the natural prostaglandins are very rapidly metabolized to less active or inactive compounds, some even within a few minutes after intravenous administration. Moreover, according to German Pat. No. 2,229,225 lactones are known (formally 9-oxo-10-oxaprostanoic acid derivatives) which are similarly prone to metabolic attack, e.g. under alkaline conditions in the gut. The 9-oxaprostanoic acid derivatives of the invention, however, are chemically and metabolically quite stable compounds which surprisingly exhibit the full spectrum of activity shown by the natural prostaglandins.